Flush valves may have a handle that, when manipulated, pushes an actuator which, in turn, opens a bypass valve within a piston in the flush valve. By opening the bypass valve, pressure above the piston drops and allows line pressure to lift the piston from its seat within the flush valve and channel water to flush a toilet, urinal or the like. While the toilet or urinal fixture is being flushed, line pressure is also directed above the piston increasing the pressure in this area. As the pressure equalizes the piston seats itself within the flush valve and stops flow therethrough.
Commercial flush valves sometimes experience problems such as water hammer and failure to shut off. Water hammer may occur if water in motion is forced to stop or change direction suddenly. This rapid change in momentum creates a surge in pressure and results in shock waves that propagate through the piping making noise.
Some plumbing codes require flush valves to have anti-backflow devices like a vacuum breaker to prevent fouling of the potable water supply in the event of backflow from the toilet or urinal fixture into the valve and the related water supply.